Biomedical subjects

Marcelo Lozada-Cassou

Publications and source records attributed to Marcelo Lozada-Cassou.

8 recordsLinked to original sources

PubMed2006

Electrolyte distribution around two like-charged rods: their effective attractive interaction and angular dependent charge reversal.

A simple model for two like-charged parallel rods immersed in an electrolyte solution is considered. We derived the three point extension (TPE) of the hypernetted chain/mean spherical approximation (TPE-HNC/MSA) and Poisson-Boltzmann (TPE-PB) integral equations. We numerically solve these equations and compare them to our results of Monte Carlo (MC) simulations. The effective interaction force, F(T), the charge distribution profiles, rho(el)(x,y), and the angular dependent integrated charge function, P(theta), are calculated for this system. The analysis of F(T) is carried out in terms of the electrostatic and entropic (depletion) contributions, F(E) and F(C). We studied several cases of monovalent and divalent electrolytes, for which the ionic size and concentration are varied. We find good qualitative agreement between TPE-HNC/MSA and MC in all the cases studied. The rod-rod force is found to be attractive when immersed in large size, monovalent or divalent electrolytes. In general, the TPE-PB has poor agreement with the MC. For large monovalent and divalent electrolytes, we find angular dependent charge reversal charge inversion and polarizability. We discuss the intimate relationship between this angular dependent charge reversal and rod-rod attraction.

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PubMed2005

The electrical double layer for a fully asymmetric electrolyte around a spherical colloid: an integral equation study.

The hypernetted chain/mean spherical approximation (HNC/MSA) integral equation for a totally asymmetric primitive model electrolyte around a spherical macroparticle is obtained and solved numerically in the case of size-asymmetric systems. The ensuing radial distribution functions show a very good agreement when compared to our Monte Carlo and molecular-dynamics simulations for spherical geometry and with respect to previous anisotropic reference HNC calculations in the planar limit. We report an analysis of the potential versus charge relationship, radial distribution functions, mean electrostatic potential, and cumulative reduced charge for representative examples of 1:1 and 2:2 salts with a size-asymmetry ratio of 2. Our results are collated with those of the modified Gouy-Chapman (MGC) and unequal radius modified Gouy-Chapman (URMGC) theories and with those of HNC/MSA in the restricted primitive model (RPM) to assess the importance of size-asymmetry effects. One of the most striking characteristics found is that, contrary to the general belief, away from the point of zero charge the properties of an asymmetric electrical double layer (EDL) are not those corresponding to a symmetric electrolyte with the size and charge of the counterion, i.e., counterions do not always dominate. This behavior suggests the existence of a new phenomenology in the EDL that genuinely belongs to a more realistic size-asymmetric model where steric correlations are taken into account consistently. Such novel features cannot be described by traditional mean-field theories such as MGC, URMGC, or even by enhanced formalisms, such as HNC/MSA, if they are based on the RPM.

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PubMed2005

Nanocap-shaped tin phthalocyanines: synthesis, characterization, and corrosion inhibition activity.

Thermal and microwave reactions between [PcSn(IV)Cl2] (1) and the potassium salts of eight fatty acids (2 a-h) led to cis-[(RCO2)2Sn(IV)Pc] compounds (3 a-h) in yields ranging from 54 to 90 %. Compounds 3 a-h were fully characterized by elemental analysis, spectroscopy (IR, UV/Vis, multinuclear NMR), and seven X-ray diffraction structures, whereby two different allotropes were observed in two cases. The two carboxylates in 3 have a cis anisobidentate binding mode, octacoordination of the tin atoms with square-antiprismatic geometry, and pi-electron-rich nanocap shapes. On account of the latter characteristics, 3 a-h compounds have anticorrosion properties. LPR and Tafel electrochemical methods were used to characterize the behavior of these derivatives in naturally aerated sour brine, which is a common environment in petroleum production and refinery operations. The measurement of the corrosion rate of carbon steel AISI 1018 in the presence of 3 a-h (500 ppm) gave efficiencies of 61-87 % for the inhibitor performance. Of the different derivatives examined, compounds 3 e and 3 h were the most effective corrosion inhibitor prototypes.

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PubMed2004

A new correlation effect in the Helmholtz and surface potentials of the electrical double layer.

The restricted primitive model of an electrical double layer around a spherical macroparticle is studied by using integral equation theories and Monte Carlo simulations. The resulting theoretical curves for the Helmholtz and surface potentials versus the macroparticle charge show an unexpected positive curvature when the ionic size of uni- and divalent electrolyte species is increased. This is a novel effect that is confirmed here by computer experiments. An explanation of this phenomenon is advanced in terms of the adsorption and layering of the electrolytic species and of the compactness of the diffuse double layer. It is claimed that the interplay between electrostatic and ionic size correlation effects, absent in the classical Poisson-Boltzmann view, is responsible for this singularity.

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PubMed2003

Overcharging in colloids: beyond the Poisson-Boltzmann approach.

A broad range of manufactured products and biological fluids are colliods. The ability to understand and control the processes (of scientific, technological and industrial interest) in which such colloids are involved relies upon a precise knowledge of the electrical double layer. The traditional approach to describing this ion cloud around colloidal particles has been the Gouy-Chapman model developed on the basis of the Poisson-Boltzmann equation. Since the early 1980s, however, more sophisticated theoretical treatments have revealed both quantitative and qualitative deficiencies in the Poisson-Boltzmann theory, particularly at high ionic strengths and/or high surface charge densities. This review deals with these novel approaches, which are mostly computer simulations and approximate integral equation theories based on the so-called primitive model. Special attention is paid to phenomena that cannot be accounted for by the classic theory as a result of neglecting ion size correlations, such as overcharging, namely, the counterion concentration in the immediate neighborhood of the surface is so large that the particle surface is overcompensated. Other illustrative examples are the nonmonotonic behavior of the electrostatic potential and attractive interactions between equally charged surfaces. These predictions are certainly remarkable and, on paper, they can have an effect on experimentally measurable quantities (for instance, electrophoretic mobility). Even so, these new approaches have scarcely been applied in practice. Thus a critical survey on the relevance of ion size correlation in real systems is also included. Overcharging of macroions can also be brought about by adsorption of oppositely charged polyelectrolytes. Noteworthy examples and theoretical approaches for them are also briefly reviewed.

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PubMed2002

Fluid-fluid correlation through a model charged membrane: analytical results.

We study the properties of electrical double layers separated by a charged plate of finite thickness. The nonlinear Poisson-Boltzmann equation is analytically solved for this system. It is shown that, for an unsymmetrically charged, narrow plate, the charged fluids at both sides of the plate are strongly correlated, and the local electroneutrality condition (LEC) is not satisfied. The LEC is satisfied only for an infinitely thick plate. Analytical expressions for the induced charge and mean electrostatic potential are given and analyzed. These findings could be relevant for the understanding of protein adsorption on membranes.

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PubMed2002

Liquid correlation across the walls in a slit pore: effect on the wetting and drying transition.

The liquid structure next to the walls of a slit pore, immersed in a model simple liquid, is studied through a liquid theory and grand canonical Monte Carlo simulations. A liquid correlation across slit walls, of finite width, is found. This correlation modifies the structure and capillary partial wetting and drying transitions of the nonhomogeneous fluid, when close to its liquid-vapor coexistence curve.

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PubMed2001

Primitive Model Electrophoresis.

The electrophoresis of a spherical macroion is calculated with the new primitive model electrophoresis (PME) theory, which incorporates the ionic size. The results are compared with the classical theory of Wiersema, O'Brien, and White. The PME mobility, as a function of the macroion's surface charge (sigma) or zeta potential (zeta), is found to depend on the ionic valence, radius and concentration, and macroion hydrodynamical radius (A); i.e., it is not universal with kappaA, as predicted by the classical theory. The mobility is very nonlinear as a function of zeta. This behavior is related to the nonlinear dependence of zeta, as a function of sigma, when ionic size is included in the theory. In the classical theory zeta is a monotonic function of sigma. Important quantitative and/or qualitative differences between PME and the classical theory are found. However, in the limit of zero ionic diameter, and/or low salt concentration, and low macroion charge, the new theory reduces to the classical theory. The agreement of the PME with experimental data is very good. In particular, the prediction of reversed mobility is corroborated by recent experimental data. Copyright 2001 Academic Press.

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